Spend 6 for attack 16, 10 for 26, 10 more for 36
Spend 2 MP for the -3 of Rapid Strike, 4 MP for -4 of swing to skull. Soooo . . . feint at 36, swing for skull at deceptive -8 for 17?

And she still has 7 MP left . . . . not sure what good it does her

Am I doing this right?

Very nearly right. For the simplified version, she has 20 MP, and gains 4 general MP (for Dodge) and 9 melee MP (for Parry) for opting not to defend. She isn't spending any MP on things that Melee MP don't allow, so that's a budget of 33 MP.

She spends 10 MP to attack at full skill. If she were just feinting, she could reduce this to 7 (for no damage), but the fact she's adding in an attack prevents that. She's taking a -3 for Rapid Strike (I assume this means she has Weapon Master), which costs 2 MP to negate. She drops the Skull penalty from -7 to -3 for another 3 MP (the better exchange rate also works for targeted attacks). She has Rapier at +12 relative to the assumed base of 14, so she can get up to +6 more than the default (+4), for a total of +10 for 10 MP. That's 25 of her MP spent, with 8 left. She's got a few options of what she could do with those 8 MP. Assuming she doesn't need any movement (AtEtB would allow her to go up to half Move normally, which would leave her with 2 MP remaining), might as well spend it on damage - that's good enough for +4, or +2/die, whichever is better (if she opted for half Move, it would only be +1, or +0.5/die, whichever is better). If using The Last Gasp, she could instead spend those 8 MP to get an HT+2 roll to recover, likely negating the AP lost from the attack. If she opted to invest 1 FP in the attack, as she would normally, that would give her and extra 4 MP, boosting damage to +3/die (just shy of x2 damage).

This leaves her at Feint 36, attack 32. She could reliable take a -16 to hit, for skill 16 and a -8 to her foe's defenses (on top of any penalty from Feint). Attack that Ends the Battle indeed.

Yes, Combat Posture results in her being better off with an All Out Attack than the normal rules would. That's part of the point. As it stands, she has the choice of what you've suggested - 1 FP, Feint 29, Attack 16, -3 to target's defenses, lose all defenses - or 1 FP, Feint 25, Attack 16, -1 to target's defenses, retain the ability to Parry at 20 (23 on a Retreat). Unless I had a character with On the Edge or similar, I certainly know I'd choose a slightly lower chance of hitting the target in order to have a much better chance of not dying if I fail. With Combat Posture, the choice isn't as clear. Going All Out means 1 FP, Feint 36, Attack 16,+3/die to damage, -8 to target's defenses, lose all defenses. Going normal would be 1 FP (spend 2 MP to negate -3 of Rapid Strike, 2 MP to drop Skull from -7 to -4), Feint 26, Attack 16, -3 to target's defenses, maintain the ability to Parry at 20 (23 on a Retreat). In that case, I'd actually be strongly tempted to go All Out.